The present invention relates to the field of waste processing and specifically to systems and methods useful in processing wastewater contaminated by volatile compounds, especially mixtures of volatile compounds and wastewater also contaminated with non-volatile materials such as salts and metal ions.
Industrial wastewater is often contaminated with harmful compounds including volatile organic or inorganic compounds as well as non-volatile harmful compounds such as detergents, salts and metal ions. As it is unacceptable to dispose of contaminated wastewater in the environment, a significant proportion of the cost for operating an industrial plant is directed to processing wastewater to neutralize the environmental and health dangers that the contaminated wastewater poses. In general, industrial wastewater is neutralized by processes that a) convert the environmentally harmful contaminants to products releasable into the environment and/or b) concentrate the environmentally harmful contaminants into as small a volume as possible for cheap internment.
It is well known in the art to remove volatile compounds from industrial wastewater or other contaminated aqueous streams by a process called steam stripping. In steam stripping, steam is externally generated and used as a stripping gas to remove volatile components from the wastewater into a vapor phase by direct contact with the wastewater inside a stripping column. The vapor phase is condensed to provide a water phase and a volatile compound phase. The volatile compound phase is separated from the water phase in a decanter vessel and incinerated or further processed for recovery while the water phase is typically combined with additional wastewater for an additional stripping step. The wastewater is stripped one or more times until relatively pure water is produced which can be released into the environment at low cost.
Steam stripping is commonly used to treat wastewater contaminated with a single volatile compound or a relatively well-defined mixture of volatile compounds, such as a specific solvent in amounts of up to about 1% by weight (about 10 gram per liter). In such cases, recovery, isolation and sale of the volatile compounds allows a reduction in the cost of operating the steam stripping unit. Thus, a steam stripping unit is advantageously used together with a single specific industrial process to recycle solvents from wastewater produced by the process.
Steam stripping is less efficient for treating wastewater containing complex mixtures of volatile contaminants, for example as may be found in dedicated waste processing plants that receive and process wastewater from multiple sources with ill-defined and changing contaminants. The volatile compound phase recovered from the vapor is a complex and ill-defined mixture of volatile compounds. In some cases, it is economically advantageous to isolate and purify specific components of the mixture, for example by distillation, but most often the components of such complex mixtures are destroyed by incineration. When the mixture includes volatile compounds that produce harmful incineration products such volatile inorganics and halogen-containing organics (e.g., chlorinated and brominated solvents) there is an added expense of scrubbers and traps for processing the wastewater.
Steam-stripping is even less suitable for processing wastewater including non-volatile contaminants, for instance wastewater including detergents, salts, buffers and metal ions in addition to volatile organic and inorganic contaminants. Steam-stripping separates the volatile contaminants from the waste water but at a higher cost: volatilization of volatile compounds from wastewater including a significant proportion of non-volatile contaminants requires more energy and there is increased sediment build up in the stripping column which requires significant maintenance. In any case, in the best case what is achieved is a minor reduction of the volume of the wastewater that still includes the non-volatile contaminants and must be further processed.
There is a need for an efficient and relatively cheap method for processing wastewater, especially wastewater including mixtures of volatile contaminants or including non-volatile contaminants together with volatile contaminants that is superior to methods known in the art.